The invention relates to a method for controlling the cuff pressure in the indirect, non-invasive, continuous measurement of the blood pressure in a finger by using a photo-electric plethysmograph in a fluid-filled pressure cuff, an electronic control circuit and an electric pressure valve, the cuff pressure being controlled by the plethysmographic signal in closed-loop operation by means of a servo-reference level, so that the arterial volume is maintained at a value to be pre-adjusted. The invention relates, furthermore, to a device to carry out said method for the indirect, non-invasive, continuous measurement of the blood pressure in a finger, which device comprises a photo-electric plethysmograph in a fluid-filled pressure cuff and associated light source and light detector, an electric pressure valve and an electronic control circuit provided with a differential amplifier, on the one input and the other input of which respectively the plethysmographic signal and a servo-reference level is supplied. Such a method and device are known from the "Zeitschrift fur die gesammte innere Medizine und Ihre Grenzgebiete" VEB Georg Thieme, Leipzig, Volume 31 (1976), pages 1030-1033.
In the method and device known from above periodical for the indirect, non-invasive, continuous measurement of the blood pressure in a finger the pressure of the fluid, e.g. air, in an inflatable cuff around the finger is controlled by means of the signal of the photoelectric plethysmograph and an electric control valve, controlled by a servo loop, in such a way that at any moment the difference between a servo-reference level or nominal value and the plethysmographic signal or real value--safe for a servo-rest error--is zero.
Such a photo-electric plethysmograph is based on the fact that in the chosen wave length range of the light it is, in a first approach, only sensitive to the light absorbing and light diffusing blood in the finger arteries, provided that the cuff pressure is sufficiently high for the other blood vessels to be empty or nearly empty, so that the total arterial blood volume will have to be constant. The artery-wall consists of elastic material so that, when the intra-arterial blood pressure changes, e.g. with the heart beat, the volume of the blood in the arteries will change also, unless the pressure at the outside of the artery, the extra-mural pressure, changes equally at each moment. When a pressure cuff of the right construction and size is put correctly around the finger, the cuff pressure will equal the extra-mural pressure so that, as a result of the described servo-control circuit in the electronic circuit, the intra-arterial pressure can be read at any moment from the cuff pressure, with a determined constant rest difference or constant transmural pressure. This constant rest difference has to be such that the cuff pressure is always lower than or at most equal to the intra-arterial pressure. When this is not the case, the finger arteries under the cuff will coincide or collapse under the influence of the extra-mural pressure being too high. In this case, it is true, the signal of the plethysmograph is also constant, and there is also a one-to-one relation between the cuff pressure on the one side and the intra-arterial pressure on the other side, but resulting from the collapse of the artery the connection with the blood pressure to the supply side is interrupted and the actual arterial blood pressure cannot be read.
From the cited periodical an initial adjustment criterium is known from which it is possible to set the servo-reference level or nominal value in the control loop in such a way that the transmural pressure is zero or practically zero without the finger arteries collapsing. The wall of the artery is then in the unloaded state, and the diameter of the artery is the unstretched diameter just before this artery collapses.